Example of DOF in head shots

I've been experimenting with various F-stop settings when taking head shots and thought I'd share the results here for anyone interested. I'm sure it's old news to many of you, but I hope some folks might benefit by seeing examples.

My goal was to figure out the best F/stop to use to keep the head in focus, but blur the background. In this case the background is some redwood trees about 20-30 feet behind the model.

I took the same shot at f/3.2, f/4 and f/5.6. The equipment was a Nikon D200 with a Tamron 28-75 F/2.8 lens.

For context - here's the shot without any editing:

Below I've cropped to just the eyes and nose. The focus point is always the bridge of the nose. You may have to zoom in a level to see the differences on the edges of the glasses.

F/3.2 (both sides of the glasses frame and edges of the face are out of focus)

F/4 (The glasses frame nearest the camera is now in focus, but the far side isn't yet)

F/5.6 (both sides of the glasses frame are in focus)

The experiment showed me that I should usually be using f/5.6 for the shots - or at least f/4 - to keep all of the head in focus.